Chemical characteristics of haze particles in Xi'an during Chinese Spring Festival: Impact of fireworks burning

Fireworks burning releases massive fine particles and gaseous pollutants, significantly deteriorating air quality during Chinese Lunar New Year (LNY) period. To investigate the impact of the fireworks burning on the atmospheric aerosol chemistry, 1-hr time resolution of PM_2.5 samples in Xi'an during the winter of 2016 including the LNY were collected and detected for inorganic ions, acidity and liquid water content (LWC) of the fine aerosols. PM_2.5 during the LNY was 167 ± 87 μg/m³, two times higher than the China National Ambient Air Quality Standard (75 μg/m³). K⁺ (28 wt.% of the total ion mass) was the most abundant ion in the LNY period, followed by SO₄ ^{2 −} (25 wt.%) and Cl⁻ (18 wt.%). In contrast, NO₃ ⁻ (34 wt.%) was the most abundant species in the haze periods (hourly PM_2.5 > 75 μg/m³), followed by SO₄ ^{2 −} (29.2 wt.%) and NH₄ ⁺ (16.3 wt.%), while SO₄ ^{2 -} (35 wt.%) was the most abundant species in the clean periods (hourly PM_2.5 < 75 μg/m³), followed by NO₃ ⁻ (23.1 wt.%) and NH₄ ⁺ (11 wt.%). Being different from the acidic nature in the non-LNY periods, aerosol in the LNY period presented an alkaline nature with a pH value of 7.8 ± 1.3. LWC during the LNY period showed a robust linear correlation with K₂SO₄ and KCl, suggesting that aerosol hygroscopicity was dominated by inorganic salts derived from fireworks burning. Analysis of correlations between the ratios of NO₃ ⁻/SO₄ ^{2 −} and NH₄ ⁺/SO₄ ^{2 −} indicated that heterogeneous reaction of HNO₃ with NH₃ was an important formation pathway of particulate nitrate and ammonium during the LNY period.